1. Field of the Invention
The present invention relates to an oil pressure controller of an automatic transmission and more particularly to an oil pressure controller for governing pressures supplied to hydraulic servos for engaging/releasing frictional engaging elements within a gear train of an automatic transmission.
2. Description of the Related Art
A plurality of frictional engaging elements are disposed in association with each element of planetary gears within a gear train of an automatic transmission. Japanese Patent Laid-open No. Hei. 1-242857 has disclosed an exemplary technology wherein private pressure governing valves are provided respectively for hydraulic servos of two different frictional engaging elements to control pressures supplied to the hydraulic servos for engaging/releasing such frictional engaging elements. In the example, a pressure governing valve for governing an oil pressure supplied to a hydraulic servo of a brake (B-3) engaged when a 1ST engine brake is attained and another pressure governing valve for governing an oil pressure supplied to a hydraulic servo of a brake (B-1) engaged when a 2ND engine brake is attained are provided separately.
Governing the oil pressures (P.sub.B-3, P.sub.B-1) necessary for engaging the respective frictional engaging elements by their private pressure governing valves, with thier solenoid pressures as signal pressures, allows a control gain of a pressure governed output (brake pressure (P.sub.B)) with respect to the signal pressure (accumu-control pressure (P.sub.ACC)) applied to the pressure governing valve, i.e. a change (dP.sub.B) of the brake pressure (P.sub.B) with respect to a change (dP.sub.ACC) of the accumu-control pressure (P.sub.ACC), to be set individually, so that the brake pressure (P.sub.B-3) which requires a low pressure may be governed accurately by lowering the control gain.
However, the prior art technology has had a problem that it is not compact because the respective private pressure governing valves have to be disposed within the valve body of the oil pressure controller. Then, the applicants of the present invention have proposed a technology for governing pressures supplied to two frictional engaging elements by one pressure governing valve to improve the compaction in the prior application (see Japanese Patent Laid-open No. Hei. 5-156395).
In the structure of the above-mentioned proposal, a control gain (dP.sub.ACC /dP.sub.B) of the pressure governing valve is fixed with respect to the both hydraulic servos, so that when it is applied to hydraulic servos wherein oil pressures necessary for engaging respective frictional engaging elements are different more or less and when the control gain is adjusted to a gain suitable for the lower pressure, the pressure supplied to the hydraulic servo cannot be raised to the engaging pressure of the frictional engaging element which requires the higher engaging pressure, causing a slip in the frictional engaging element. Further, when the control gain is adjusted to a gain suitable for governing the higher pressure (P.sub.B-1) as shown in FIG. 5, the control gain becomes too large when the pressure (P.sub.B-4) supplied to the hydraulic servo of the frictional engaging element whose engaging pressure is lower is controlled and an output pressure (dP.sub.B) varies significantly with a slight change of the signal pressure (dP.sub.ACC), so that the variation of the output pressure with respect to the variation of the signal pressure increases, degrading the pressure governing accuracy.
The present invention has been devised in view of such circumstance and it is an object of the present invention to provide an oil pressure controller of an automatic transmission which is capable of supplying engaging pressures to hydraulic servos of frictional engaging elements which require different engaging pressures with a good pressure governing accuracy while improving the compaction by unitizing the pressure governing valve.